Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

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238 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems Data Unit Identifier Information Object This is the structure with SQ = 1, which implies a sequence of information elements within one information object. There is just one information object address, which is the address for the first information element. Figure 8.25 Simplified structure of ASDU with SQ=1 For each defined type identification, there are either one or two forms of the ASDU depending on whether one or both variable structure codes are allowed. The ASDUs are made of components that have all been introduced and described in the preceding sections. These are summarized below. ASDU components: • Data unit identifier – Type identification – Variable structure qualifier – Cause of transmission – Common address of ASDU • Information object – Information object address – One or more information elements – Time tag if used • Additional information objects (if SQ = 0) – Information object address – One or more information elements – Time tag if used The following sub-sections show the ASDUs organized by type numbers and groups. To avoid unnecessary repetition, for each ASDU only the information objects are shown. The reader must remember that the complete ASDU has the data unit identifier prior to the information object or objects. Also, note that only one information object is shown for each of SQ = 0 and SQ = 1. In the case of SQ = 0, which implies a sequence of information objects the reader must be aware that although only one information object is presented, multiple information objects may be used to form the ASDU.

Fundamentals of IEC 60870-5 239 In the case of SQ = 1, which implies a sequence of information elements within one information object, there is of course only the one information object. For these note that there is only one information object address and one time tag, if used. The information object addresses for sequential information elements within the ASDU are obtained by incrementing from the address of the first information element. Thus for the i th information element, the address is given by IOA + I − 1. The time tag therefore applies to all of the information elements within the ASDU. 8.7.1 Process information in monitor direction Type 1 Single-point without time INFORMATION OBJECT TYPE: 1 CODE: M_SP_NA_1 DESCRIPTION: Single point information without time tag VALID WITH SQ: 0, 1 Information object for SQ = 0 (sequence of Information Objects) Information Object Address IV NT SB BL SPI SIQ Information object for SQ = 1 (Sequence of Information Elements) Information Object Address IV NT SB BL SPI SIQ Information element 1 IV NT SB BL SPI SIQ Information element N Valid cause of transmission codes Background scan Spontaneous Requested Return of information caused by remote command Return of information caused by local command Interrogated by station interrogation Interrogated by group G interrogation, G= Notes SIQ is single point information with quality